My father in law has a 425 and it has been having this problem since about last summer. I haven't looked at it yet but plan to soon. Just want to throw this out there and see if anybody has noticed it before. He can start it up and run it full throttle for about 10 minutes, then it starts running rough. He can sometimes slowly put the choke on and get it to smooth out and run it full throttle for the rest of the day with no problems. Other times it will die and it might not start right away. Once it does start is runs fine. The little foam piece under the gas cap fell off into the tank at some point and that could be plugging the pump strainer but doesn't make sense for how it can run fine later. He was thinking the thermostat might be going bad. It never gets to the red he says. I guess it could be sticking and then pop open causing it to run at the correct temp for the rest of the time. I was thinking something to do with the carb since it smooths out when he chokes it some. Figured it was running lean so restricting the airflow helped it out. Any ideas would be appreciated. Thanks.